The gearboxes installed at present in self-propelled machines of the lawn mower type comprise a frame within which is enclosed a primary shaft disposed generally parallel to a secondary shaft and transmitting to this latter its rotative movement by means of an assembly of gears and a movable key ensuring the change of speed. Stopping the drive of the primary shaft means permitting changing the speed is achieved by a principal clutch device disposed outside the frame. This clutch device is generally a friction clutch mechanism constituted by a transmission belt disposed in an annular groove of a rotating pulley, this transmission belt being by variation of its tension maintained or not within the groove so as to drive or not the primary shaft. This type of clutch, in addition to its size, is difficult to adjust. Thus, it is noted that in such mechanisms, the shaft is sometimes driven while the drive belt is slackened. Moreover, as this clutch is not a progressive clutch, it requires in order to avoid shocks, the installation of an auxiliary clutch. Such a transmission device is for example described in U.S. Pat. No. 4,841,794. In this patent, the speed change shaft and the output shaft are two coaxial shafts adapted to be connected together by means of an auxiliary clutch with teeth or balls. The principal clutch and the auxiliary clutch are connected by a clutch lever.
French Patent FR-A-92.3171 describes a speed change gearbox comprising a primary shaft, a secondary shaft and an intermediate shaft. The clutch as described in this patent is a clutch of the claw-type actuated by means of a fork. Clutching takes place by means of the keys contained in the groove of the primary shaft and spaced by the truncated conical piston which acts under the influence of one or several longitudinal springs disposed within the primary shaft. These keys lock, during their displacement, a speed change gear in engagement with the first pinion or a second pinion coming to rest within recesses provided in said pinion. In locked position, the shaft, the keys, the springs and the speed change gear form a rigid entity which turns as a unit. This rotative movement is transmitted to the intermediate shaft which itself transmits it to the output shaft. This gearbox is cumbersome and requires three shafts to be able to operate.